Optical coating

ABSTRACT

Optical components (16), particularly germanium components, are provided with a coating on exposed surface (15) within a vacuum chamber (17) by production of a glow discharge plasma containing carbon and another depositable element from feedstock gases which are fed to the chamber (17) via mass-glow rate controllers (9, 10, 11). The mass flow rate of the feedstock gases is maintained substantially constant at predetermined levels during respective time intervals to provide a multilayer coating of predetermined characteristics. Typically the coating has at least one first layer which is amorphous hydrogenated germanium carbide, at least one second layer which is amorphous hydrogenated germanium and at least one third layer which is amorphous hydrogenated carbon, these layers being ordered such that each second layer is bounded on each side by a first layer and the third layer is bounded on one side by a first layer, the other side of the third layer forming the exposed surface of the coating.

This invention relates to a process for coating optical components, toapparatus for coating optical components in a glow discharge plasma andto coatings for optical components.

There are a number of known techniques for coating optical componentsbut hitherto it has not proved practical to provide a coating which ismultilayer, very hard, wear-resistant, substantially transparent toinfrared radiation and substantially anti-reflective to infraredradiation. Primarily this arises because in a multilayer coating thethickness of each layer is of critical importance and existing methodsof measuring thickness have proved expensive, complex and inadequate.

The present invention comprises a process for coating optical componentscomprising the steps of providing a vacuum chamber containing a cathode,arranging a substrate to be coated on the cathode, providing in thechamber a glow discharge plasma containing carbon and another element tobe deposited as a coating, feeding gases containing carbon and saidother element to the chamber during growth of the coating, andcontrolling the mass flow rate of said gases to be substantiallyconstant at predetermined levels during respective predetermined timeintervals, whereby the substrate is provided with a multilayer coatinghaving predetermined characteristics.

Preferably the mass flow rate is maintained constant to within ±10% ateach predetermined level.

Preferably the vacuum pressure within the chamber during growth of thecoating is controlled to be substantially constant at predeterminedlevels during said respective predetermined time intervals. Convenientlythe vacuum pressure is maintained constant to within ±25% at eachpredetermined level.

Preferably the electrical bias voltage applied to the cathode duringgrowth of the coating is controlled to be substantially constant atpredetermined levels during said respective predetermined timeintervals. Conveniently the bias voltage is maintained constant towithin ±15% at each predetermined level.

Preferably the temperature of the surface of the optical component beingcoated is controlled to be substantially constant at predeterminedlevels during said respective predetermined time intervals. Convenientlythe temperature is maintained constant to within ±20° C. at eachpredetermined level.

By virtue of the present invention where mass flow rate is controlled ithas proved possible to dispense with layer thickness measuring deviceswhilst achieving accurate layer thicknesses.

The gases, the mass flow rates of which are controlled, may for examplebe Germane (so that the said other element is Ge) and a hydrocarbon gassuch as butane. Silane may be used in place of Germane in suitablecases. The optical component may be made of Germanium or Silicon, forexample.

The present invention further comprises apparatus for coating opticalcomponents, comprising a vacuum chamber containing a cathode, means fordelivering a flow of feedstock gases to the chamber, a mass flow ratecontrol arrangement for said means, and means for establishing a glowdischarge plasma in said chamber.

The present invention further provides a coating for an opticalcomponent, said coating comprising a multiplicity of layers including atleast one first layer which is amorphous hydrogenated germanium carbide,at least one second layer which is amorphous hydrogenated germanium, andone third layer which is amorphous hydrogenated carbon, the layers ofsaid multiplicity being arranged such that each second layer is boundedon both sides by a first layer, and the third layer is bounded on oneside by a first layer, its other side forming the exposed surface of thecoating.

Preferably each first layer has a refractive index of about 2.8, eachsecond layer has a refractive index of about 4.1, and the third layerhas a refractive index of about 2.0, the refractive indices being atsaid predetermined wavelength.

In a first example there is only one second layer, two first layers andthe aforesaid third layer. In a second example there are two secondlayers, three first layers and the aforesaid third layer.

The present invention further comprises an optical component made ofgermanium and having the aforesaid coating adherent to at least oneoptical surface of the germanium component.

Preferably the layers of the coating are formed by plasma enhancedchemical vapour deposition (i.e. a glow discharge plasma) from germane(GeH₄) and Hydrocarbon (such as Butane (C₄ H₁₀)) feed gases.

Preferably the temperature of the surface of the optical component beingcoated is maintained at a first predetermined level during deposition ofeach of the first and second layers and at a second predetermined levelduring deposition of the third layer, the second predetermined levelbeing substantially less than the first predetermined level.Conveniently the first predetermined level is 350° C. and the secondpredetermined level is 200° C.

Preferably an electrical bias voltage is applied to the opticalcomponent during deposition and the bias voltage is maintained at afirst predetermined level during deposition of each of the first andsecond layers and at a second predetermined level during deposition ofthe third layer, the second predetermined level being substantiallygreater than the first predetermined level. Conveniently the firstpredetermined level is 500 volts and the second predetermined level is1000 volts.

Preferably the chamber vacuum pressure during deposition of the firstand second layers is maintained at a first predetermined level and ismaintained at a second predetermined level for the third layer thesecond predetermined level being substantially less (further fromatmospheric) than the first predetermined level. Conveniently the firstpredetermined level is 50 m Torr and the second predetermined level is 8m Torr.

Embodiments of the present invention will now be described by way ofexample with reference to the accompanying drawings, in which:

FIG. 1 illustrates apparatus for coating optical components inaccordance with the present invention;

FIG. 2 illustrates the transmittance and absorptance characteristics ofa thin germanium substrate coated on both faces with a coating inaccordance with the present invention;

FIG. 3 illustrates the transmittance and reflectance characteristics ofa thin germanium substrate coated on one face with a coating inaccordance with the present invention and on its other face with a known`internal surface` coating;

FIG. 4 illustrates the transmittance characteristic of a thin germaniumsubstrate coated on one face with a prior art coating and on its otherface with the same known `internal surface` coating as in FIG. 3.

As is shown in FIG. 1 of the drawings apparatus 20 for coating opticalcomponents comprises a vacuum chamber 17 having a cathode 18 forsupporting an optical component 16 to be coated on its exposed surface15. The cathode 18 is heated by a unit 17 and controlled in temperatureby a controller 12 so that surface 15 is maintained at a desiredtemperature. Additionally AC power at radio frequencies (RF) is suppliedto the cathode 18 by a power supply unit 6 via an impedance matchingunit 5 (for maximum efficiency). The required level of vacuum isestablished within the chamber 17 by a high vacuum pump 4, motorisedthrottle valve 3, capacitance manometer 1 and vacuum controller 2connected in closed loop. Gases are delivered into the chamber 17 bythree pipelines with respective closed-loop mass flow controllers 9, 10,11 via a common ON/OFF valve 8.

In one example of a coating deposited on a substrate surface 15 thecoating has four discrete layers and the parameters applicable to theapparatus 20 are set forth in Table I. Layers Nos. 1 and 3 are composedof amorphous hydrogenated germanium carbide: Layer No. 2 is composed ofamorphous hydrogenated germanium; and Layer No. 4 is composed ofamorphous hydrogenated carbon. Prior to the coating being deposited thesurface 15 of the substrate 16 is cleaned by sputtering with Argonapplied via mass flow controller 9. Layers Nos. 1 and 3 are deposited byfeeding butane (C₄ H₁₀) via mass flow controller 10 and Germane (GeH₄)via mass flow controller 11. Layer No. 4 is deposited by feeding onlybutane (C₄ H₁₀) via mass flow controller 10. The respective levels offlow rate, vacuum pressure, cathode bias voltage, time interval andtemperature of surface 15 are identified in Table I together with thedesign optical thickness of each layer and the design refractive indexeach at the predetermined wavelength of 10 μm.

In operation of the apparatus 20 the mass flow rates are controlled towithin ±10%, the bias voltage is controlled to with ±15%, the vacuumpressure is controlled to within ±25% and the temperature is controlledto within ±20° C. as a result of which it has been found that thecharacteristics achieved by the resultant coating is within 1% of itsdesign values over the three portions of the infrared waveband ofparticular interest, namely 2.05 to 2.2 μm, 3 to 5 μm, and 8 to 11.5 μm.The characteristics of the coated substrate are shown in FIG. 2 togetherwith the design characteristics (both faces of the germanium disc havingthe same coating).

The coating referred to in Table I is designed to have good durabilitywhen exposed to environmental conditions, i.e. to be an `externalcoating` and when tested against the U.K. standard (TS 1888) providedthe results tabulated in Table II. Further tests at more severe levelsthan TS 1888 produced the results shown in Table III. Exceptionaldurability is thereby demonstrated.

In practice the coating referred to in Table I would be applied only toone surface of an optical component (which might form a window havingtherefore only one surface exposed to environmental conditions), theother surface of the component being coated with a known `internalcoating` of which various are known. FIG. 3 illustrates the opticalcharacteristics of such an optical component in the form of a thingermanium disc and for the purposes of comparison FIG. 4 illustrates acharacteristic of a similar disc coated with the same `internal coating`and a prior art `external coating`. It will be observed that the coatingof the present invention (i.e. as per Table I) provides vastly superiorcharacteristics.

The coating referred to in Table I is a four layer coating and, ifexpanded to a six layer coating as set forth in Table IV exhibits stillfurther improved optical properties notably a 3% reduction inreflectance over the 8 to 11.5 μm waveband. It will be noted that TableIV is relatively concise in the interests of simplicity, but layer Nos.1, 3 and 5 are each identical in all respects to layers Nos. 1 and 3 ofTable I, whilst layers Nos. 2 and 4 are each identical in all respectsto layer No. 2 of Table I, and layer No. 6 is identical in all respectsto layer No. 4 of Table I.

                                      TABLE I                                     __________________________________________________________________________                                        SUB-                                                      FLOW RATE      BIAS STRATE               REFRAC-                              (STANDARD cm.sup.3 /                                                                    PRES-                                                                              VOLT-                                                                              TEMPER-                                                                             TIME   OPTICAL TIVE                                 MINUTE    SURE AGE  ATURE INTERVAL                                                                             THICKNESS                                                                             INDEX                LAYER                                                                              MATERIAL                                                                             GAS i.e. SCCM (m Torr)                                                                           (V)  (°C.)                                                                        (minutes)                                                                            (λ/4 at 10                                                                     (at 10               __________________________________________________________________________                                                             μm)               Substrate                                                                          Germanium                                                                            Ar  5         20   500  350   15     --      4                    Sputter                                                                       Clean                                                                         1    H:α-Ge:C                                                                       C.sub.4 H.sub.10                                                                  5         50   500  350   6.1    0.116   2.8                              GeH.sub.4                                                                         10                                                            2    H:α-Ge                                                                         GeH.sub.4                                                                         15        50   500  350   3      0.137   4.1                  3    H:α-Ge:C                                                                       C.sub.4 H.sub.10                                                                  5         50   500  350   32     0.689   2.8                              GeH.sub.4                                                                         10                                                            4    H:α-C                                                                          C.sub.4 H.sub.10                                                                  5          8   1000 200   22     0.416   2                    __________________________________________________________________________

                  TABLE II                                                        ______________________________________                                                       NUMBER OF     NUMBER                                           TS1888 TEST    FACES TESTED  PASSED                                           ______________________________________                                        Adhesion       4             4                                                Humidity       4             4                                                Salt Water Immersion                                                                         3             3                                                (7 days)                                                                      Eraser         3             3                                                Wiper (10,000 wipes)                                                                         3             3                                                Constant Low   3             3                                                Temperatures                                                                  Icing/Frosting 3             3                                                Driving Rain   3             3                                                HCI Immersion Test                                                                           1             1                                                ______________________________________                                    

                  TABLE III                                                       ______________________________________                                                       NUMBER OF     NUMBER                                           OTHER TEST     FACES TESTED  PASSED                                           ______________________________________                                        Salt Water Immersion                                                                         1             1                                                (94 days)                                                                     Wiper (100,000 wipes)                                                                        3             3                                                Wiper (200,000 wipes)                                                                        3             2                                                Thermal Shock  4             4                                                (40° C. to 0° C.)                                               Extreme Thermal Shock                                                                        2             2                                                (100° C. to -196° C.)                                           ______________________________________                                    

                  TABLE IV                                                        ______________________________________                                                           OPTICAL     REFRACTIVE                                                        THICKNESS   INDEX                                          LAYER  MATERIAL    λ/4 AT 10 μm                                                                    (AT 10 μm)                                  ______________________________________                                        Substrate                                                                            Germanium   --          4                                              1      H:α-Ge:C                                                                             0.0424     2.8                                            2      H:α-Ge                                                                              0.699       4.1                                            3      H:α-Ge:C                                                                            0.072       2.8                                            4      H:α-Ge                                                                              0.188       4.1                                            5      H:α-Ge:C                                                                            0.654       2.8                                            6      H:α-C 0.468       2                                              ______________________________________                                    

What is claimed is:
 1. A process for coating infrared transmissiveoptical components with a multilayer coating which is wear resistant,substantially transparent to infrared radiation, and substantiallyanti-reflective to infrared radiation, wherein the thickness of eachcoating layer is measured in fractions of a quarter wavelength at 10 μm,said process includinglocating a component to be coated on a cathodewithin a vacuum chamber, establishing vacuum pressure within thechamber, elevating the temperature of the component, applying a desiredbias voltage to the cathode, and forming in the chamber a glow dischargeplasma with gaseous deposition material fed into the chamber, wherebyconstituents of the plasma material are caused to be deposited on thecomponent, said process further comprising the steps of sequentiallyfeeding different gaseous deposition materials into the chambercontinuously during respective sequential time intervals to formsuccessive different layers of deposition on the component and for eachsuch layer regulating the mass flow rate of the gaseous depositionmaterials to within ±10% of a predetermined flow rate level; the vacuumpressure of the chamber to within ±25% of a predetermined vacuumpressure level; the cathode bias voltage to within ±15% of apredetermined bias voltage level; and the component temperature towithin ±20° C. of a predetermined temperature level, the final layerbeing amorphous hydrogenated carbon deposited from gaseous depositionmaterial in the form of a hydrocarbon gas, and the duration of each saidtime interval being of the order of minutes whereby to provide that eachlayer has a thickness of the order of fractions of a quarter wavelengthat 10 μm.
 2. A process as claimed in claim 1, wherein the first suchlayer deposited onto the substrate is amorphous hydrogenated germaniumcarbide deposited from a mixture of germane and hydrocarbon gas fed asdeposition materials, the second such layer deposited onto the substrateis amorphous hydrogenated germanium deposited from germane gas fed asdeposition material, and the third such layer deposited onto thesubstrate is amorphous hydrogenated germane carbide deposited from amixture of germane and hydrocarbon gas fed as deposition materials,wherein the predetermined levels of vacuum pressure, bias voltage andcomponent temperature remain unchanged during the respective first,second and third time intervals during which said first, second andthird layers are deposited to thickness values determined by therespective time intervals and predetermined levels of flow rate, andsaid final layer is deposited on said third layer.
 3. A process asclaimed in claim 1, wherein the first such layer deposited onto thesubstrate is amorphous hydrogenated germanium carbide deposited from amixture of germane and hydrocarbon gas fed as deposition materials, thesecond such layer deposited onto the substrate is amorphous hydrogenatedgermanium deposited from germane gas fed as deposition material, thethird such layer deposited onto the substrate is amorphous hydrogenatedgermane carbide deposited from a mixture of germane and hydrocarbon gasfed as deposition materials, and between said third layer and said finallayer there is interposed at least one further layer of amorphoushydrogenated germanium carbide deposited from a mixture of germane andhydrocarbon gas fed as deposition materials and at least one furtherlayer of amorphous hydrogenated germanium deposited from germane gas fedas deposition material, the predetermined levels of vacuum pressure,bias voltage and component temperature remaining unchanged during therespective time intervals during which said first, second, third andeach further layers are deposited and differing from the predeterminedlevels of vacuum pressure, bias voltage and component temperatureestablished during deposition of said final layer.
 4. A process asclaimed in claim 1, wherein the predetermined level of vacuum pressureduring deposition of said final layer is less than that applied duringdeposition of all preceding layers, the predetermined level of biasvoltage during deposition of said final layer is greater than thatapplied during deposition of all preceding layers, and the predeterminedlevel of component temperature during deposition of said final layer isless than that applied during deposition of all preceding layers.
 5. Aprocess as claimed in claim 1, wherein the respective predeterminedlevels during the respective time intervals are as set forth in Table 1.